A roller coaster has 3 trains with 7 rows per train. Riders stand in rows of 6, for a total of 42 riders per train. The operators of the coaster recorded the number of riders on each train during a run. On the first train, the operators reported that 6 rows were filled. On the second train, all 7 rows were filled. On the third train, 4 rows were filled. How many rows were empty on the first train? How many additional riders would it take to fill those empty rows?